An exhaust system of an internal combustion engine is usually equipped with means for cleaning and treating the exhaust gases removed from the internal combustion engine. It may be necessary in this connection to introduce a liquid educt into the exhaust gas stream, to evaporate it in same and to mix it with the exhaust gas. It may be necessary, for example, to add a fuel to the exhaust gas upstream of an oxidation catalytic converter in order to bring about an exothermal reaction of the fuel in the oxidation catalytic converter. The heated exhaust gas stream can then be used downstream of the oxidation catalytic converter to heat an additional exhaust gas treatment means, for example, another catalytic converter or a particle filter, to operating temperature or to regeneration temperature. Furthermore, SCR systems are known, which operate with selective catalytic reaction and are equipped with an SCR catalytic converter, which absorbs NOx from the exhaust gas stream.
To make possible the most space-saving design of the exhaust system possible, the oxidation catalytic converter and the SCR catalytic converter may be arranged in parallel next to each other. The outlet-side end of the oxidation catalytic converter and the inlet-side end of the SCR catalytic converter are connected to one another in this case via a part of the exhaust system called a funnel or mixing chamber such that the exhaust gas flow is deflected by 90° each after the oxidation catalytic converter and in front of the SCR catalytic converter.
A suitable reducing agent, for example, ammonia or urea, preferably an aqueous urea solution, is added as an educt to the exhaust gas stream in the area of the mixing chamber upstream of the SCR catalytic converter. The ammonia will then bring about a conversion of the nitrogen oxides present into nitrogen and water in the SCR catalytic converter.
It applies to all educts added in the liquid form to the exhaust gas stream that the desired effect can only be obtained satisfactorily if sufficient evaporation of the educt as well as sufficient mixing of the gaseous educt with the exhaust gas stream can take place between the site at which the liquid educt is introduced and a site at which the educt is consumed. The mixing and/or evaporating means mentioned in the introduction are used for this purpose, which are arranged in the flow path of the exhaust gas between the site at which the educt is introduced and the site at which the educt is consumed. However, only the relatively short mixing chamber between the oxidation catalytic converter and the SCR catalytic converter is available for the evaporation and mixing of the reducing agent in the above-described embodiment of the exhaust system, which makes complete evaporation and mixing of the reducing agent before entry into the SCR catalytic converter difficult.
In addition, the reducing agent is usually added via a nozzle, which introduces the reducing agent as an expanding, conical jet into the exhaust gas stream. Part of the reducing agent is not consequently moving exactly in the direction of flow in the exhaust gas stream, but with a direction component in the direction of the wall of the exhaust system. Since the mixing chamber may have a relatively flat flow cross section, this may cause droplets to hit the wall of the mixing chamber and form a film on the wall before the reducing agent is evaporated. It is very difficult to return the reducing agent from such a film on the wall into the exhaust gas stream and to evaporate and mix it. The formation of a film on the wall is additionally also facilitated by the fact that the reducing agent is introduced into the exhaust gas stream directly behind the oxidation catalytic converter in order to have the longest possible flow path available for evaporation and mixing of the reducing agent. However, as was mentioned above, the exhaust gas flow is deflected by 90° in this area, as a result of which centrifugal forces may additionally develop, which may throw the droplets onto the wall of the mixing chamber.